In the field of equipment including an image display apparatus that requires video signal processing, the video signal processing is generally executed by digital signal processing due to widespread use of chips and devices called DSPs (digital signal processors).
FIG. 6 shows an arrangement of an image display apparatus having an LCD (liquid crystal display) as a display device as an example of equipment for executing video signal processing by the DSP provided therewith.
A DSP 1 shown in the figure is composed of, for example, one chip or one device and executes necessary signal processing to an input digital video signal by a signal processing unit 11 formed in the chip or the device. The signal processing executed here is digital signal processing. The DSP 1 outputs the digital video signal, which is subjected to the signal processing by the signal processing unit 11, to the outside from a terminal T1 after it is converted into an analog video signal by a D/A converter 12.
In this case, the terminal T1 of the DSP 1 is connected to a terminal T4 of an LCD drive circuit 2 composed of one device likewise, for example, the DSP 1, thereby the analog video signal output from the terminal T1 of the DSP 1 is input to the LCD drive circuit 2 through the terminal T4.
The LCD drive circuit 2 creates a drive signal for driving an LCD 3 for display based on the analog video signal input thereto and outputs the drive signal from a terminal T5. In this case, the terminal T5 is connected to a terminal T6 of the LCD 3 as a display device, and the drive signal is input to the LCD 3 from the terminal T6.
The LCD 3 drives a pixel cell in response to the drive signal input thereto. With this operation, the LCD 3 displays an image according to the video signal.
Incidentally, it is assumed that the image display apparatus arranged as shown in, for example, FIG. 6 must be modified so that a novel video signal processing function can be added thereto. It is first contemplated to remake the DSP 1 to cope with the modification. However, remaking it takes a cost such as development cost, a manufacturing cost, and the like. Accordingly, when the modification is not executed on a large scale, there is a disadvantage in that an effect such as an appeal to users and the like obtained by the modification does not worth the cost increased by remaking the DSP 1.
In this case, there is employed a method of mounting an external circuit (chip, device) that corresponds to the novel video signal processing function. Further, when the external circuit is composed of an analog circuit for executing analog video signal processing, a disadvantage arises in that a circuit size is increased and dispersion and the like of a signal level are also increased. Thus, it is preferable to arrange the external circuit so as to execute digital signal processing.
FIG. 7 shows an image display apparatus on which mounted is an external circuit (chip, device) arranged to execute the digital signal processing as described in the latter method. Note that, in FIG. 7, the same portions as those in FIG. 6 are denoted by the same reference numerals, and the explanation thereof is omitted.
In the image display apparatus shown in the figure, a signal processing block 4 as an external digital signal processing circuit (chip, device) is interposed between a DSP 1 and an LCD drive circuit 2.
Since a terminal T2 of the signal processing block 4 is connected to a terminal T1 of the DSP 1, an analog video signal is input to the terminal T2 after it is subjected to signal processing by the DSP 1.
As an internal arrangement of the signal processing block 4, first, the analog video signal input through the terminal T2 as described above is converted into a digital video signal by an A/D converter 21 so that it can be subjected to digital signal processing internally. Then, video signal processing corresponding to a specific function is executed by the digital signal processing executed by a signal processing unit 22. Then, the digital video signal, which is subjected to the signal processing as described above, is converted into an analog video signal by a D/A converter 23 so that it can be input to an LCD drive circuit 2 and then output from a terminal T3. The terminal T3 is connected to a terminal T4 of the LCD drive circuit 2, thereby the analog video signal is input to the LCD drive circuit 2 (refer to Japanese Unexamined Patent Application Publication No. 10-336547).
Incidentally, the signal processing block 4 in the image display apparatus shown in FIG. 7 includes cells of the A/D converter 21 and the D/A converter 23 so that it cope with an input/output of the analog video signal while executing the video signal processing therein by the digital signal processing. Further, in the image display apparatus shown in FIG. 7, a cell of a D/A converter 12 is also included in the DSP 1. Accordingly, a system of the image display apparatus shown in FIG. 7 has the cells of three sets of the A/D converters and D/A converters in its entirety.
As an actual matter, it is known that the cells of the A/D converters and the D/A converters of these devices have dispersion (error) in input/output signal levels (data values in a digital signal).
Dispersion of the cells of the A/D converters and the D/A converters is guaranteed so that it is within a predetermined range. However, when the number of the cells of the A/D converters and the D/A converters connected to each other in series increases as shown in FIG. 7, an error of an overall data value (signal level) increases. When the error increases as described above, an original dynamic range cannot be effectively utilized because, for example, a data value (level) tends to overflow (excessively input) or a signal level is made too small.
What has been described above will be explained with reference to FIGS. 8A, B, and C.
First, FIG. 8A shows a case in which the dynamic range (maximum output level) of the D/A converter 12 in the DSP 1 is the same as the dynamic range (maximum input level) of the A/D converter 21 of the signal processing block 4.
The data value of an input signal S1 (output from a signal processing unit 11 in FIG. 7) of the D/A converter 12 is set to a level Ldr that corresponds to the dynamic range DR of the A/D converter 21. Then, a signal S2, which is obtained by converting the input signal S1 into an analog signal by the D/A converter 12, has the level Ldr because the dynamic range of the D/A converter 12 is the same as that of the A/D converter 21.
That is, this case is in such a state that an ideal dynamic range is secured in which an input signal having a maximum value is kept as it is without overflowing.
In contrast, FIG. 8B shows a case in which the dynamic range (maximum output level) of the D/A converter 12 in the DSP 1 is larger than the dynamic range (maximum input level) of the A/D converter 21 of the signal processing block 4 as a relation to the dispersion of error.
In this case, since the D/A converter 12 has the larger dynamic range, the signal S2, which is obtained by converting the input signal S1 having the level Ldr into the analog signal, is output in a level La higher than the level Ldr as shown in the figure.
In this case, even if the signal S2 is input to the A/D converter 21, since the level of the signal S2 exceeds the dynamic range of the A/D converter 21, the data value of a signal output from the A/D converter 21 overflows.
Further, FIG. 8C shows a case in which the dynamic range (maximum output level) of the D/A converter 12 in the DSP 1 is smaller than the dynamic range (maximum input level) of the A/D converter 21 of the signal processing block 4 as a relation to the dispersion of error.
In this case, since the D/A converter 12 has the smaller dynamic range, the signal S2, which is obtained by converting the input signal S1 having the level Ldr into the analog signal, is output in a level Lb smaller than the level Ldr.
The signal S2 has a small level in correspondence with a difference of levels Ldr-Lb with respect to the dynamic range of the D/A converter 12 regardless that the level of the signal S2 must be originally Ldr. That is, the dynamic range cannot be sufficiently secured.
As described above, the dispersion of error of the cells of the D/A converters and the A/D converters appears in a state that the dynamic range is made improper as described above, this state appears as a phenomenon, for example, deterioration of solarization.